The present invention relates to an improved means for recovering an optical signal for an optical receiver in an optical fiber asynchronous distribution network.
In an optical fiber bus distribution network, when an optical receiver receives information from a remote source, it is necessary for the receiver to first determine the decision threshold or decision trigger level for incoming signals to be decoded. The threshold or trigger level for an optical detector which utilizes electronics for decoding is generally a voltage which represents either a logic high or a logic low. For such a detector to operate effectively the clock frequency or clock rate at which the light pulses representative of information being sent has been generated must be determined so that the receiver can "lock on" to this clock frequency and accordingly accurately read all the light pulses being sent. If the receiver does not lock onto the appropriate threshold level and actual clock frequency, then the receiver is likely to output significant quantities of bit errors.
As used throughout herein, the "clock frequency" represents the number of the light pulses which are sent per unit time (e.g. 100 megabits/sec), and does not refer to the wavelength or frequency of the light used to generate any given pulse (e.g. typical wavelengths being 820 nm, 1300 nm, and 1500 nm).
Prior art attempts at locking onto the appropriate threshold and actual clock frequency by which data is generated and sent comprise dedicating a predetermined relatively large number of bits at a beginning of a packet of data for these functions, e.g. these bits contain no real data and simply constitute a plurality of logic highs and logic lows in a series long enough for the detector or receiver to lock onto the actual clock frequency for the data packet being sent. Thereafter, the receiver continues to track the clock frequency by continuously detecting light pulses from the source generating the light pulses.
Though such a procedure is acceptable in a synchronous network having only one source and one clock where the clock frequency once determined remains relatively constant over time, such a procedure is disadvantageous in an asynchronous bus network where data is being sent by and received from numerous remote terminals separated geographically since the particular clock frequency and threshold level for each packet of data sent by each terminal is unique to its particular clock and therefor must be determined by the receiver each time a packet of data from a different source is to be detected. In this situation, dedicating a relatively large number of bits for clock and threshold level recovery for each packet results in a disadvantageous large bit overhead for the network which results in less of the network bandwidth being available for actual data transfer.